The Idle of Car Engines: Two Operating Points?

When I was a boy, my dad would go out and warm up the car in the winter. The car would idle fast. Later, he’d then tap the accelerator, and the idle would slow to a more relaxed pace. Why was this? What was determining the idle of the engine before and after? What did tapping on the gas do? (These cars didn’t have fuel injection.)

Now older and wiser, I WAG the tap opened the choke (or is it the throttle?) to allow in a leaner or richer fuel-to-air mixture? And, why don’t today’s cars require (a) step on the gas before starting the engine to introduce some gas and (b) today’s cars only seem to idle at one speed, or am I mistaken?

Last, what did/does the accelerator control, anyhow? Fuel pump rate of pumping? and/or opening the choke? throttle? These terms are used so interchangeably! Please clarify if your explanation is in terms of older cars, today’s cars, or both. And, if it should matter, please specify if your reply is specific to any one brand of car or American design vs. some European country’s design vs. Japanese, etc.

Please set me on the right road (ha-ha)! Thanks! Happy Motoring! And remember, you car trust your car to the man who wears the star… - Jinx, a boy at the end of the Esso age in the US (funny to still see Esso in Canada!)

Carbuerated cars used an “automatic choke” that employed a bimetal spring to set the choke/throttle. A cold car ran at higher revs. After it had warmed up, a tap on the gas pedal would allow the choke to revert to its normal operating position and the throttle to return to its idle speed setting. Cars don’t do this anymore because modern engines are fuel-injected and electronically controlled.

As SavageNarce mentioned cars used to have automatic chokes. here is the deal, more fuel or less air is needed on a cold engine. Rich mixture is needed cold to get the fuel to to burn. Stepping down on the gas pedal cold allowed the choke plate to close blocking most of the incoming air, at the same time it moved a little lever under the throttle stop screw holding the throttle part way open to increase the idle. So at start up your engine would have a rich mixture, and a high idle, this would allow the engine to warm up. Once the engine had started to warm up a tap on the throttle would reset the choke to the warm position, mixture would revert to normal.

Again as SavageNarce mentioned today’s cars have a whole host of sensors that determine the correct mixture based on air temp, coolant temp, altitude, barometric pressure (and maybe a few others). The engine then starts with as close as possible to the correct mixture. A high idle may not be necessary. Although on the cars I teach on, the new models have what is called a Cat start function. The car will idle high (about 1700 RPM for about 20-30 seconds to heat up and light off the catalytic converter.

On all gas engines I am familiar with the speed of the engine is controlled by regulating the amount of air introduced into the engine. The accelerator pedal controls the position of the throttle plate. The throttle plate is a disc(s) in the intake that blocks most of the ai rwhen closed, and as the throttle is open allows more air into the engine. At wide open throttle the disc is edgewise to the flow of air and blocks almost no air. Control of the throttle plate may be either mechanical (cable or linkage) of electrical (fly by wire). No, throttle and choke are not interchangeable terms. On computer controlled engines (read fuel injected or modern engines) there maybe an auxiliary channel for air to enter the engine controlled by a electronic valve, called an idle air control valve (IACV in technician speak) this valve is controlled by the engine management system to regulate the idle speed. In gear, out of gear, A/C on or off, the idle speed is regulated by the computer to maintain a constant idle. On fly by wire cars, there is no IACV, idle speed is controlled by moving the throttle plate open and closed to maintain the proper idle.

Hope this helped, if you have further questions, feel free to ask.

The gas/accelator/throttle pedal opens the throttle (a spring closes it). The throttle is a flap in the passageway of air going into the engine for combustion. Thus your accelerator control is directly controlling how much air can be drawn into the engine’s cylinders. The fuel system – carburetor or fuel injection – is designed to provide a proportionate amount of fuel so the engine will have air and fuel together to burn.

Fuel must be vaporized to burn efficiently, and cold engines don’t vaporize it as well as warm engines, so there is some system to add some extra fuel for starting and running a cold engine. While a smaller percentage of it is vaporized, with the greater total amount of fuel the result is that about the same amount of vaporized fuel is available.

Most carburetors, and a tiny handful of older fuel injection systems, used a choke to add extra fuel when cold. A choke is a flap like the throttle, located in the air passageway where its restriction of airflow doesn’t cause a proportionate lessening of fuel delivered, thus having the overall effect of increasing the amount of fuel going into the engine. Note that throttle and choke are related terms, and have the same meaning when talking about restricting airflow through a person’s throat. In automotive use, the terms are used quite specifically for their different respective functions, even though their mechanisms are essentially identical.

Now, with that background: Older carbureted cars needed to idle at a significantly higher speed while cold, in order to deal with the extra gas provided by the choke. So along with the choke there’s a fast idle mechanism. It has either a series of steps or a ramped design. to provide the greatest idle increase with the choke full on, and progressively less increase as the choke is working its way to the off position during warmup.

The choke pulls the fast idle cam along with it when going on (cold engine). But while the choke spring would automatically try to do this as the engine got cold, the full motion would be blocked by the fast idle cam hitting the idle screw, which would be left in its normal warm idle position. So, you floor the gas pedal on the cold engine and voila: A) the fast idle cam can move into its cold position, B) the choke moves to its on position, and C) the accelerator pump gives a priming squirt of gas. Turn the key, and the engine starts and runs at fast idle.

The choke gradually goes to its off postion as the car warms up. However, the fast idle cam will stay where it was until the gas pedal is opened, because the idle screw is pressed against it. If you blip the pedal every minute during warmup, the fast idle comes down gradually. If you leave it alone for several minutes, it comes down in a bigger jump. Tapping on the pedal releases the pressure of the idle screw on the fast idle cam, and lets the cam go to where it “wants” to be.

On modern fuel injected cars, the same things are accomplished in very different ways. Fuel mixture (e.g., extra gas for starting cold) and idle speed are controlled by the electronic control system (computer). There’s no need to step on the gas pedal to activate it for a cold start, because there’s no mechanism that has to move into position. The idle speed is higher when cold, but not as much higher as with carbureted cars, because fuel injection systems can very effectively do “just enough” of everything. The transition from fast idle to warm idle is gradual and smooth without noticeable jumps.

A. Today’s cars do not require this because the fuel system is pressurized by an electric pump as soon as the ignition is turned on. If you (carefully!)wait by the rear of a car while a friend turns the ignition slowly, you may be able to hear the pump engage.
Over the years, car makers have used a variety of diffrent systems to achieve better cold starts and smooth idle operation, such as cold start injectors and idle air control valves/motors.

B. You are mistaken. Most, if not all, modern cars run at a higher RPM, if only for a moment, after cold starts. There are simply too many permutations to list, but the basic Electronic Fuel Injection (EFI) design uses a computer that controls the rate of fuel delivery (“map”, “curve”) based on inputs from several sensors, including temperature sensor(s). Once sensor input indicates that the engine is “warmed up” the computer can run in “closed loop” mode and adjust the fuel more exactly, allowing the engine to run more efficiently. A basic system uses a temperature sensor, Mass Airflow sensor on the intake, and oxygen sensor in the exhaust.

Furthermore, new cars, particularly “low emissions” vehicles, employ a second catalytic converter, or “pre-catalyst” closer to the exhaust manifold. These pre-catalysts must be heated up very quickly in order to realize maximum emissions control, and cars with this feature usually run at a noticeably higher RPM initially.

Whether old or new, Japanese, European, or American, the throttle (gas pedal, accelerator) actually controls the amount of air entering the engine. Another sensor, called a Throttle Position Sensor, provides input to the computer that is used to control the rate of fuel delivery through th efuel injectors. The only exception to this rule would be ultramodern cars like some BMWs, in which everything is computer controlled, even the actual throttle opening, and engine speed is actually controlled by the valves.

In any case, throttle opening is unrelated to the amount of fuel pumped by an electric fuel pump. On older cars with mechanical pumps running off the camshaft, opening the throttle affects the rate of pumping, by extension, because the pumping rate is related to engine RPM, but the rate of fuel pumping is not used to regulate engine speed- any well designed fuel system is capable of pumping more gas than the engine needs.

Whoops, whooshed by Gary T and Rick.

good answers all.